DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
695 
fishes previously described, only in every instance the bone retains its areolar 
character, and the investing periosteum appears to have more the character of a 
fibro-cartilage than of a simple fibrous membrane. The ossifying surfaces exhibit a 
film of chondriform bone like that seen at the bases of the neurapophyses of the 
vertebra; whilst laminae of bone, resembling that seen in the centrum, successively 
shoot into the cartilage, the intervening portions of which are absorbed as they 
advance leaving long parallel cancelli, abounding in fat-cells. It is obvious, that a 
slight difference exists between the chondriform bone contained in the cartilage at 
the extremities of these elongating laminae and that of the laminae themselves, as well 
as of the osseous cylinder in which they are contained : the latter tissues approximate 
more to the nature of true membraniform bone, and yet they cannot be regarded as 
such. As far as I am able to comprehend their curious structure, all the bones of 
the {Salmon appear to be of a chondriform character ; but whilst in some cases, where 
the bone is designed to be a temporary growth, the calcareous matter is deposited in 
the unaltered intercellular substance of cartilage, in others, where the bone is de- 
signed to be permanent, the cartilage is converted into a fibro-cartilage previous to 
calcification. At the same time that tl>e bones ai’e increasing at their extremities in 
the way already described, a prolongation of the same altered cartilage or fibro- 
cartilage, containing fusiform cells, and investing their exteriors as a periosteum, is 
increasing their thickness by successive external additions, in which, as in the other 
parts of the skeleton, the presence of cells in the fibro-cartilaginous matrix leads to 
the existence of corresponding areolae in the bone. 
During the examination of the other osseous fishes referred to in the preceding 
pages, the question frequently suggested itself to me as to whence the fibrous 
membrane which lined the cavities encroaching upon the cartilage is derived; is 
it an altered condition of the surface of the cartilage itself, or after the latter was 
partially absorbed did the cavity become lined with an entirely new structure, which 
calcification was subsequently to convert into membraniform bone? Whatever was 
the origin of the tissue, it appeared to be of the same character as the perichon- 
drium and the periosteum. In endeavouring to trace a line of division between 
the cartilage and the perichondrium I usually failed ; whilst at the bases of the neu- 
rapophyses of the Pike, where the external periosteum and the subjacent internal 
fibro cartilage came into contact, there appeared to be a commingling of the two 
tissues, rather than a mere juxtaposition. Many of these circumstances combine 
to produce a conviction, that the cavities about to be transformed into cancelli were 
not lined by an entirely new membrane, but by one formed out of the fibrillated sub- 
stance of the cartilage. So far as I comprehend the growth of the bones of the Salmon, 
they appear to countenance this opinion. We find that the cartilage, prior to its 
conversion into what in other allied fishes would become membraniform bone, 
assumes in a very marked manner a fibrous character, without entirely losing its cha- 
racteristic cells. At the same time, the growths, which in other allied osseous 
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